Quantitative Treatment of the Function of Plastoquinone in Photosynthesis

I. The function of plastoquinone PQ in photosynthesis has been measured by absorption changes in the UV at 265 nm. These measurements have been refined by the application of the very sensitive repetitive pulse technique. The reduction and oxidation of plastoquinone in chloroplasts have been measured 1. in short flashes, 2. in long flashes and 3. in double flashes. The results can be described by the empirical equations 1 — 13.

II. A molecular mechanism for these reactions of plastoquinone is presented. Two electron chains are arranged parallel to each other in such a way that two chlorophyll-a_II are in contact with a twin of PQ-PQ (see fig. 12).

Firstly, plastoquinone is reduced by chlorophyll-a_II into PQ^-— —PQ^-. Secondly, the semiquinone PQ^- — PQ^- dimutates in a first-order reaction into PQ⁼ and PQ. Thirdly, the free diffusion within the plastoquinone pool results at light reaction I in a conmutation of PQ⁼+PQ into two single PQ^- molecules. These are oxidized via intermediates (cytochrome-f etc.) by chlorophyll-a1⁺. The conmutation is the rate limiting step of the overall reaction.

III. The theoretical treatment of the postulated molecular mechanism in II results in equations 14 — 31. These equations agree quantitatively with the experimental equations 1 — 13.

The postulated semiquinone twin PQ^- — PQ^- has been attributed to the as yet chemically unknown substance X-320 with absorption changes in the UV at 320 nm.

The reaction mechanism of PQ regulates the kinetics and reaction patterns of the whole electron transfer system from H₂O to the endstanding electron acceptor.

The rate of reduction of the electron acceptor ferricyanide and the rate of O₂-production from H₂O have been calculated from the PQ-mechanism. These rates are in quantitative accordance with the measured rates of ferricyanide reduction and O₂-production.